What was the mark that the Lord put on Cain so no one would kill him (Gen. 4:15)?

We are not told any details about the mark God put on Cain. The word translated “mark” is used of the sign of the rainbow in Genesis 9:12-13 and the sign of circumcision in Genesis 17:11. Those two things were a physical sign, so it is likely the sign on Cain was a physical sign or mark. Also, it seems that the mark on Cain was something visible to those who would try to attack him in vengeance, gives weight to the idea of a physical mark.

The word “mark” is translated from the Hebrew word “oth”. This word is translated in the King James Version as follows: ensign x 2, mark x 1, miracle x 2, sign x 60 and token x 14. It is often used in connection with the signs done in Egypt at the time of the Exodus.